> Only use 90 GW when 90 GW is available, and run the aluminum smelters undercapacity when there isn't the solar power to run them at full capacity.
That gets you a few percent for the things that work like that, but most of the load on the power grid isn't able to do that. You can't turn off the heat in the winter, which is going to be an even larger proportion of the load in the future than it is now as we transition from fossil fuel heating systems to electric. People demand hot water. Refrigeration is a significant load but you turn it off and your food spoils.
Electrifying transportation is also a mixed blessing here. It helps smooth out the day/night load because you can charge them whenever generation exceeds demand, but after a couple days you can't put it off anymore and then you've just got more unsuppressible demand.